1. Field of the Invention
The present invention relates to a phase shifter for shifting a signal in phase and particularly to a hybrid type phase shifter.
2. Description of the Related Art
In the past, various type phase shifters have been used. They are important with development of electrical communication.
For example, satellite communication requires an antenna for tracking a satellite. Particularly, a satellite tracking antenna which is mounted on a moving objects such as motorcar or the like is required to be reduced in size and electric power consumption. It is thus believed that the satellite tracking antenna on the moving object is perferably a phased array antenna. The phased array antenna is required to control the phase for each of antenna elements which form an array. Therefore, the phase shifter becomes one of very important components for the phased array antenna.
Phase shifters which are used in such a phased array antenna and the like include digital phase shifters which are adapted to change the amount of phase shift from one to another by on-off controlling a switch. The digital phase shifters are known to be of loaded line type, switched line type, hybrid type and so on. Among them, the hybrid type phase shifter is preferred since it has a relatively simple structure for providing any amount of phase shift.
On the other hand, the digital phase shifters utilize a switch for selecting the amount of phase shift, which may be a diode type or field effect transistor (FET) type switch. FET type is believed to be particularly suitable for use in such an antenna as mounted on the moving object such as motorcar or the like, since the FET type switch has a necessary power lower than that of the diode type switch on the order of several figures and may include a simplified bias circuit without any capacitor for cutting off DC.
In other word, antenna systems on motorcars or other moving objects require an electrical power consumption as low as possible since the limited capacity of battery must be effectively utilized. The antenna systems, which are used in the motorcars or other moving object, must be of a construction as simple as possible since they are used under severe conditions such as vibrations associated with the running vehicles, intensive changes of temperature and so on.
It is therefore preferred that a hybrid type phase shifter having a FET switch is used as a phase shifter mounted on the moving object.
On example of the conventional hybrid phase shifters with a switch for selecting the amount of phase shift is shown in FIG. 17. The hybrid phase shifter comprises a three dB hybrid element 10 in which an input signal is divided into two output signals of equivalent magnitude, and two phase shift regulating circuits 12.
The hybrid element 10 includes an input terminal 10a receiving an input signal and an isolation terminal 10b providing an output signal. The hybrid element 10 also includes a coupling terminal 10c and a through terminal 10d. The two phase shift regulating circuits 12 are connected with the coupling and through terminals 10c, 10d in the hybrid element 10.
The two phase shift regulating circuits 12 are of the same construction which comprises a first line 12a having one opened end and a second line 12b cascade connected between the other end of the first line 12a and a switch 12c.
The functional principle of this phase shifter will be described in connection with FIG. 18 which is a Smith chart.
There is first considered the reflection coefficient .GAMMA. in the switch 12c, which is one viewed from a reference plane C to the switch side. Ideally, the reflection coefficient .GAMMA. is equal to -1 when the switch 12c is ON and equal to one when the switch 12c is OFF. If the switch 12c is of FET type, however, it includes an induction component and a capacity component. As shown in FIG. 18, thus, the reflection coefficient .GAMMA. is in a position .GAMMA. Con which is substantially equal to -1 and shifted clockwise due to the induction component if the switch 12c is ON. On the other hand, if the switch 12c is OFF, the reflection coefficient .GAMMA. is in another position .GAMMA. Coff which is substantially equal to one and shifted clockwise due to the capacity component.
It is secondly considered the reflection coefficient .GAMMA. which is viewed from a reference plane D including the second line 12b (characteristic impedance Z0) to the switch side. If the characteristic impedance of the line 12b is equal to 50 .OMEGA., the reflection coefficient .GAMMA. is in the respective positions .GAMMA. Don and .GAMMA. Doff when the switch 12c is ON and OFF. in which positions the reflection coefficient .GAMMA. in the reference plane C is rotated to the side of power source (clockwise) by the electrical length of the line 12b while maintaining its magnitude constant.
It is further considered the reflection coefficient .GAMMA. which is viewed from a reference plane E including the first line 12a. The reference coefficient .GAMMA. is rotated on a constant conductance circle toward the side of power source (clockwise) to a position .GAMMA. Eon or .GAMMA. Eoff in either time when the switch 12c is ON or OFF. In other words, the reflection coefficient .GAMMA. viewed from the reference plane E when the switch 12c is ON and OFF is rotated on the constant conductance circle which is determined depending on the position of the reflection coefficient .GAMMA. viewed from the reference plane D.
Therefore, the reflection coefficient .GAMMA. on ON and OFF in the switch 12c can be determined by varying the first and second lines 12a, 12b in length and other parameters. As a result, a difference .phi. between phases when the switch 12c is ON and OFF becomes the amount of phase shift at the output terminal 10b. When the switch is turned on or off, the amount of phase shift in the phase shifter can be changed from one to another by setting the first and second line 12a, 12b at predetermined lengths.
However, the aforementioned phase shifter constructed in accordance with the prior art has the following problems:
(A) The amount of phase shift can be set only by adjusting both the first and second lines 12a, 12b. This adjustment is very difficult. More particularly, the adjustment of the reflection coefficient .GAMMA. on the constant conductance circle by regulating the length of the first line 12a should be combined with the adjustment of the length of the second line 12b. It is extremely difficult to find a proper combination of length between the first and second lines 12a, 12b.
(B) Generally, the FET switch has less property in its ON state than that of the OFF state. This fact is not well considered in the conventional phase shifter as described. Thus, the phase shifter will have a loss substantially increased from that of the OFF state. Such an increased difference of loss between the ON and OFF states of the phase shifter is very detrimental for the application of the aforementioned phased array antenna.
(C) Although the functional principle of the prior art has been described as to a single frequency, it must be measured throug the entire frequency band actually used therein. The aforementioned phase shifter has a possibility in which the frequency band is extremely narrowed depending on the case. It is very difficult to find under what condition the frequency band can be widened. Although some other configurations in addition to the aforementioned phase shifter are known in the art, none of them could overcome the above three problems and set any desired amount of phase shift.
In order to overcome all the problems in the prior art, an object of the present invention is thus to provide a digital phase shifter which can set any desired amount of phase shift very simply.